Allow whitespace in curly bracket interpolations (e.g. "%{ foo }").

For someone who follows the [Ruby Style Guide](https://github.com/bbatsov/ruby-style-guide#pad-string-interpolation), the lack of support for whitespace padding inside I18n's interpolation is quite surprising. In fact, I spent quite a bit of time recently hunting for a reason why interpolation wasn't working, when it was just that I was including whitespace inside my brackets. I feel like this violates the principal of least surprise.

This pull request resolves the problem for curly brackets, though I left the %<n>.d interpolation alone. I'd be happy to do same for angle brackets if you would find that prudent.